Focus: Magnetic Shepherd

Cleverly arranged electromagnetic fields could form a ‘one-way door’ that allows electrons through on one side but repels them from the other.

General Atomics

Tamed plasma. Electrons zipping around inside a fusion reactor such as this one might be controlled by a “one-way door” that allows electrons through on one side but repels them from the other.

Tamed plasma. Electrons zipping around inside a fusion reactor such as this one might be controlled by a “one-way door” that allows electrons through on one side but repels them from the other. [Credit: General Atomics]×

Electrons in a plasma are like sheep in a field–they gambol around and sometimes need a good prod to nudge them in the right direction. In the 14 November PRL, a team describes a seemingly magical method for providing that prod efficiently for electrons orbiting around a fusion reactor. They propose using electric and magnetic fields to create a one-way wall that lets electrons through in one direction but blocks them in the other. The technique could also lead to a new way of trapping charged particles. The concept is reminiscent of “Maxwell’s Demon,” a character dreamed up by a physicist studying thermodynamics in the 19th century.

In a doughnut-shaped fusion reactor called a tokamak, researchers use magnetic fields to keep the superhot plasma from splattering against the walls and prematurely halting the reactor. They produce part of this containment field by driving some of the plasma’s electrons around the ring in one direction. But the usual methods for generating this current require pumping large amounts of radio wave energy into the plasma, much of which is wasted heating some electrons more than necessary.

Nat Fisch of Princeton University and his colleagues propose driving the current by adding energy in only a small region, rather than everywhere around the torus. The idea combines two types of fields: a thin wall of oscillating electromagnetic fields that slices vertically through the doughnut, and a static magnetic field. Electrons prefer to avoid regions of strong and oscillating fields, so the wall alone would fling them out. But the static magnetic field makes the wall act like a one-way door.

To see how it works, imagine an electron approaching the wall. The static magnetic field–which is perpendicular to the wall–makes the particle whirl around in small orbits as it advances a bit forward with each cycle, tracing out a helical path. Near the wall, the frequency of this “cyclotron” motion matches that of the oscillating fields, which point within the plane of the wall. The oscillating magnetic field within the wall then provides perfectly timed kicks at specific points in the orbit of the electron. These kicks push all electrons in the same direction, regardless of which side they came from, because they are effectively synchronized in their orbits.

The electromagnetic wall acts like the character 19th-century physicist James Clerk Maxwell discussed in connection with the Second Law of Thermodynamics. He imagined a gas-filled box with a partition down the middle. A small creature opens and closes a tiny door in the partition and allows only fast-moving molecules to come through to one side and slow ones to the other. Fisch and his colleagues say their proposal doesn’t amount to a true Maxwell Demon because the electrons absorb energy when they hit the wall of fields.

The team cautions that the idea may be difficult to implement practically in a tokamak, although they also imagine using it to trap particles between two parallel walls of electromagnetic fields. Thomas Antonsen of the University of Maryland in College Park says the new approach could be very efficient in a tokamak. If the static field is sufficiently localized, electrons could be reflected or transmitted using only a small amount of energy, he says. But he adds that the proposal needs to be examined more closely. “Plasmas always find ways of confounding those making predictions.”